import java.util.Scanner;

public class Main3{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        double r = in.nextDouble();
        Pentagon pentagon = new Pentagon();
        double area = pentagon.getArea(r);
        System.out.printf("%.3f\n", area);
    }
}

class Pentagon{
    public double getArea(double r){
        return (5*Math.pow(2*r*Math.sin(Math.PI/5),2)/(4*Math.tan(Math.PI/5)));
    }
}